An example of a conventional optical disc focus control device is disclosed in Patent Document 1.
Patent Document 1 discloses a device which detects a previously-recorded signal for adjustment, and adjusts a focus control system so as to make the detected signal maximum.
FIG. 8 is a block diagram illustrating the configuration of the focus control device disclosed in Patent Document 1.
In FIG. 8, z1 denotes a light source, z2 denotes a semi-transparent mirror, z3 denotes a convergent lens, z4 denotes a recording medium on which a signal for adjustment has previously been recorded, z5 denotes a focus drive circuit, z6 denotes a divided photodetector, z7 denotes a focus error detection means, z8 denotes a reproduction signal processing means, z9 denotes a focus balance adjustment means, z10 denotes a focus balance change means, z11 denotes a focus control means, z12 denotes a focus drive means, and z13 denotes a path of laser light from the light source to the divided photodetector. An optical pickup al is an element comprising the light source z1, the semi-transparent mirror z2, the convergent lens z3, the focus drive circuit z5, and the divided photodetector z6.
The focus control in this device will be described. A light beam which is applied to the convergent lens z3 with its light axis being shifted is converged onto the recording medium z4, and the reflected beam is separated by the semi-transparent mirror z2 to be applied to the divided photodetector z6. Since this light beam is applied to the convergent lens z3 with its light axis being shifted, the position of the reflected beam is moved according to the vertical movement of the recording medium z4. So, the movement of the reflected beam is detected by the divided photodetector z6, and the movement amount of the reflected beam (focus error) from the position where the beam is focused on the recording medium z4 is detected by the focus error detection means z7. Next, a movement amount of the convergent lens z3 for minimizing the focus error is obtained by the focus control means z11, and a drive signal for moving the convergent lens z3 by the obtained movement amount is outputted from the focus drive means z12. The focus drive circuit z5 and the convergent lens z3 connected to the focus drive circuit z5 are driven by the drive signal so that the light beam is constantly in the predetermined converged state on the recording medium z4.
Next, the focus control system adjustment method of this device will be described. A signal having a predetermined frequency is previously recorded in a spiral on the recording medium z4. When a light beam is applied to the recording medium z4 and focus control is performed under the state where the recording medium z4 is rotated, the reproduction signal processing means z8 which receives a sum signal from the divided photodetector z6 provides a reproduction signal output z14 as shown in FIG. 2. In FIG. 2, the abscissa shows the time axis, and z14 denotes the reproduction signal output.
FIG. 3 shows a spot of the light beam on the recording medium z4. In FIG. 3, z15 denotes signal recording tracks on the recording medium, z16 denotes a non-recorded portion between the tracks, and z17 denotes a spot of the light beam on the recording medium. When an offset is given to the focus error by the focus balance change means z10, a movement amount of the convergent lens z3 corresponding to the offset is outputted from the focus control means z11, and thereby the convergent lens z3 is moved.
FIG. 4 shows the relation between the position of the convergent lens z3 and the reproduction signal z14, wherein (a) to (c) show the states where the convergent lens positions are A to C, respectively. The output power of the reproduction signal Z14 varies depending on the spot diameter of the light beam, and when the light beam is focused (when the convergent lens position is B) as shown in FIG. 4(b), i.e., when it is correctly focus-controlled, the spot diameter becomes minimum and thereby the amplitude of the reproduction signal output becomes maximum. Since data can be accurately reproduced from the recording medium z4 when the reproduction signal output is maximum, the position of the convergent lens z3 is adjusted in the optical disc device so as to make the reproduction signal output maximum before reproduction of data from the recording medium z4.
FIGS. 5 and 6 show the convergent lens position adjustment methods A and B by which the reproduction signal output is maximized. Initially, the procedure of the adjustment method A will be described with reference to FIG. 5.
In FIG. 5, the amplitude of the reproduction signal at the convergent lens position A(1) before adjusted is measured in step A1. Next, the convergent lens position is shifted to A(2), and the reproduction signal amplitude at A(2) is measured. Since the amplitude at A(2) is smaller than the amplitude at A(1), the convergent lens position is shifted to A(3) which is in the direction opposite to A(2).
Next, in step A2, after the amplitude at A(3) is measured, the convergent lens position is shifted to A(4) and the amplitude is measured. Since the amplitude at A(4) is larger than the amplitude at A(3), the process goes to step A3.
In step A3, the convergent lens position is shifted to A(5) and the amplitude is measured. Since the amplitude at A(5) is larger than the amplitude at A(4), the process goes to step A4.
In step A4, the lens position is shifted to A(6) and the amplitude is measured. Since the amplitude at A(6) is smaller than the amplitude at A(5), adjustment is ended with A(5) being the lens position after adjusted.
Next, the procedure of the adjustment method B will be described with reference to FIG. 6.
Initially, in step B1, the reproduction signal amplitude at the convergent lens position B(1) before adjusted is measured. Next, the convergent lens position is adjusted to B(2) at which the reproduction signal amplitude becomes maximum, in the same procedure as that of the adjustment method A shown in FIG. 5.
Next, in step B2, the lens position is shifted from B(2) in the direction opposite to that in step B1 to find a lens position B(3) at which the reproduction signal has an amplitude that is obtained by subtracting a threshold value from the amplitude at the lens position B(2).
In step B3, the lens position is shifted from B(2) in the same direction as that in step B1 to find a lens position B(4) at which the reproduction signal has an amplitude that is obtained by subtracting the threshold value from the amplitude at the lens position B(2).
In step B4, the adjustment is completed with the lens position B(5) intermediate between B(3) and B(4) being the lens position after adjusted.
When the adjustment method A is compared with the adjustment method B, the adjustment method A can complete adjustment in a shorter time relative to the adjustment method B, but it causes an increase in variation of the adjusted lens position when plural times of adjustments are performed.
FIG. 7 shows the relation between the reproduced signal amplitude and the convergent lens position. Assuming that the minimum value of the reproduction signal amplitude which is required to accurately reproduce the data from the recording medium z4 is z18, the convergent lens position at which the reproduction signal amplitude becomes z18 or more is in the range including z19 to z20. This range is called a margin z21. The margin z21 has the individual variability due to variations in the processes of manufacturing the convergent lens z3, the focus drive circuit z5, and the division photodetector z6. Conventionally, an adjustment method by which variation in the convergent lens position that is adjusted by a focus control device having the narrowest margin falls within the margin is used for all focus control devices.
Patent Document 1: Japanese Published Patent Application No. Hei. 2-64920